The present invention relates in general to reducing the effects of common source inductance in discrete semiconductor power switching devices, and, more specifically, to inverter drive systems for electrified vehicles using discrete power switching devices with fast switching times and low losses.
Various types of semiconductor power switching devices have been introduced for high-voltage, high-power electronic switching applications such as power MOSFETs and insulated gate bipolar transistors (IGBTs). Due to the semiconductor die sizes and heat sinking requirements, these devices are usually contained in discrete packages mounting one or more switching devices, e.g., in a “transistor-outline” (TO) package style. The discrete power switching device packages are usually mounted to a printed circuit board that also carries ancillary electronics associated with the switching application.
Electric vehicles, such as hybrid electric vehicles (HEVs) and plug-in hybrid electric vehicles (PHEVs), employ such power switching devices to construct inverters for electric machines to provide traction torque and regenerative braking torque. A typical electric drive system includes a DC power source (such as a battery pack or a fuel cell) coupled by contactor switches to a variable voltage converter (VVC) to regulate a main bus voltage across a main linking capacitor. A first inverter is connected between the main bus and a traction motor to propel the vehicle. A second inverter may be connected between the main bus and a generator to regenerate energy during braking to recharge the battery through the VVC.
The inverters include power switching devices (most typically IGBTs) connected in a bridge configuration. An electronic controller turns the switches on and off via gate driver circuits in order to invert a DC voltage from the bus to an AC voltage applied to the motor, or to invert an AC voltage from the generator to a DC voltage on the bus. In each case, the inverters are controlled in response to various sensed conditions by varying the frequency and duty cycle at which the power devices are switched on and off.
The inverter for the motor pulse-width modulates the DC link voltage to deliver an approximation of a sinusoidal current output to drive the motor at a desired speed and torque. PWM control signals applied to the gates of the IGBTs turn them on and off as necessary so that the resulting current matches a desired current. The IGBTs and their reverse-recovery diodes have associated switching losses which must be minimized in order to limit loss of efficiency and creation of waste heat.
Common source inductance refers to an inductance shared by the main power loop (i.e., the drain-to-source or collector-to-emitter power output of the device) and the gate driver loop (i.e., gate-to-source or gate-to-emitter) in a power switching device. The common source inductance carries both the device output current (e.g., drain to source current) and the gate charging/discharging current. The voltage induced across the common source inductance modifies the gate voltage in a manner that limits the on/off switching times and increases switching losses. In a typical switching module, there are various contributors to the common source inductance which arises as a parasitic inductance associated with device packaging and printed circuit board (PCB) traces. The relative placement of current paths and the use of various structures to separate and/or block inductive coupling have been used to reduce the magnitude of the parasitic inductance that is created.
Despite the known practices, the common source inductance for discrete power devices introduced by packaging (e.g., TO-247 and TO-220) can still be as high as 10 nH. With new generations of power devices (e.g., CoolMOS, SiC, and GaN devices) becoming faster and faster, the common source inductance dramatically limits the switching speed and increases switching losses.